LONDON – As America's friends and allies look on in astonishment at the all-but-certain prospect of a contest between Hillary Clinton and Donald Trump in November's US presidential election, they need to do more than just wring their hands. They must hope for the best but prepare for the worst.

The crucial point about the 2016 election is not just that a reality-TV star and property magnate who has never held elected office has emerged as the presumptive Republican candidate. It is the enormous difference that a victory by Trump would make for the rest of the world compared with a victory by Clinton.

In every US presidential election in modern times, America's friends and allies have had their private preferences. But never before have the Democratic and Republican candidates been as different as chalk and cheese. There was no unbridgeable gulf between Reagan and Carter, or Clinton and Bush, or Bush and Gore, or Obama and McCain. There is between Trump and Clinton.

To the rest of the world, Clinton represents continuity, and Trump means dramatic change. Just how dramatic cannot be known, but the normal assumption that candidates play to their party's core supporters during the primary season but then tack to the center for the general election cannot be relied on in Trump's case. His is an abnormal candidacy.

That is why preparation makes sense. Trump confirmed in his speech on foreign policy to the Center for the National Interest in Washington, DC, on April 27 that "America First" would be the overriding theme of his administration. He would reject multilateral trade deals and institutions, take a much tougher line on illegal immigration, and forge a new approach to defense and security alliances.

Trump declared in that speech that he wants the United States to be "predictably unpredictable," but he also made it clear that he won't abandon his basic position. Allies will have to pay more for their defense. And they can expect tough measures by his administration if they have an enduringly large bilateral trade surplus with the US. Regional deals, such as the 1994 North American Free Trade Agreement (NAFTA) between the US, Mexico, and Canada, are a "total disaster" that have tied America's hands. So the best assumption is that they would be scrapped.

So how can friends and allies prepare for a President Trump? Discreetly, of course. But the author of the 1987 bestseller The Art of the Deal would surely agree that sound preparation is the essence of striking a good bargain. If Trump finds evidence of it after having won the White House, he would likely admire his counterparts for it, even if secretly.

There are two sorts of things that friends and allies can and should do to prepare for the worst. One is to make themselves stronger and thus better able to stand up to a bully. The other is to shore up their alliances and friendships with one another, in anticipation of an "America First" rupture with old partnerships and the liberal international order that has prevailed since the 1940s.

A weak Japan and a fractious collection of 28 countries in the European Union would be a tempting target for President Trump. A Japan that had, over the next 12 months, truly embraced the growth-enhancing strategy of liberalization that has often been promised by Prime Minister Shinzo Abe would be in a stronger position. So would European countries that dropped their obsession with fiscal austerity and used public-investment programs to kick-start growth and reduce unemployment.

Such moves, which are needed in any event, would make it easier to start on the task of building stronger alliances – which may well become essential.

If a Trump administration seeks to scrap NAFTA, Canada and Mexico will need to make common cause. If it chooses to discard the Trans-Pacific Partnership (TPP) trade agreement negotiated by the Obama administration with 12 Asia-Pacific economies, those countries, perhaps led by Japan or Australia, must be ready to carry on the deal, or something like it, among themselves. (Clinton has also turned against the TPP, but this can be assumed to be merely tactical; in Trump's case, no such assumption is warranted.)

A similar story applies in Europe. To avoid being pushed around by Trump over trade or security, members of the EU and NATO must be prepared to stick together. That may mean being ready to spend more on their own defense – a demand by Trump that is not unreasonable. It will also mean being sufficiently united to avoid being picked off singly by an American bully.

And yet European solidarity is fraying, to say the least, thanks to the migrant crisis and the economic aftermath of the 2008 financial crash. On June 23, British voters could make the situation far worse if they vote in their referendum to leave the EU. To strengthen Britain, and the EU itself, in anticipation of Trump, they would be well advised to vote to stay in.

Asia has not been known for its solidarity. It has depended, perhaps excessively, on American influence to balance its rivalries. Japan, for example, has close ties with Southeast Asian countries but no formal security relationships with them. Both Japan and its nearest neighbor, South Korea, have longstanding security treaties with the US, but are hostile to each other.

Given the possibility of trade wars, currency wars, and a renunciation of long-held security alliances within the next 9-12 months, it is time to put regional solidarity ahead of old enmities and the forces of fragmentation. America's friends and allies need to start preparing for a less friendly America.